Laser marking is a marking method for printing a letter, a number, a trademark, a bar code, or the like, or forming an image directly on a base material utilizing a laser light.
A marking system utilizing a laser light has the following major characteristics:
(1) it is a non-contact marking method, and therefore, marking can be performed at a high speed on a base material with an arbitrary shape; and
(2) an ink is not used, and therefore, marking has excellent abrasion resistance and alteration is difficult, and also volatilization of an organic solvent or the like does not occur, and therefore, the environmental impact is low.
Due to this, at present, marking is transferred from a conventional ink system to a laser marking system in many industries.
In laser marking, a CO2 laser, a YAG laser, a YVO4 laser, a green laser, or the like is used, however, in order to enable fine printing, mainly a YAG laser is preferably used.
However, most major resin molded materials poorly absorb a laser light (typically, a near-infrared light at a wavelength of 1064 nm from YAG). Due to this, it is often difficult to obtain marking with sufficient quality in terms of visibility and definition, and there are not a few resins on which printing cannot be performed at all. This is a large restriction on application of laser marking.
As a known technique for improving the visibility of laser marking on a resin molded material, a technique in which various additives which absorb a laser light are blended in the material is known.
For example, PTL 1 discloses that particles of mixed oxides of tin and antimony having a particle diameter of 10 to 70 nm are added to a molded material (base material) as an additive for laser marking.
The principle of marking is that when the above-mentioned particles in a resin molded material absorb a YAG laser light, the light is converted to heat, and a portion surrounding the additive is carbonized to form a contrast with a portion which is not irradiated with the laser.
PTL 2 describes that laser marking can be performed on a thermoplastic plastic containing a pigment obtained by covering a flaky substrate such as a mica flake or an SiO2 flake with tin dioxide doped with antimony, arsenic, bismuth, copper, gallium, germanium, or an oxide thereof.
However, the methods of these literatures are configured such that the additive acts by absorbing a laser light to convert the light to heat and carbonize the peripheral resin.
Therefore, the quality of marking largely depends on the ease of carbonization of a resin or the shape of a resin composition which is a marking target. That is, in the case where a base material resin is hardly carbonized, marking with favorable visibility cannot be obtained, and also in the case where marking is performed on a resin molded material in the shape of a thin film such as a coating material or a film, the thickness of the resin to be carbonized is insufficient, and therefore, a laser light penetrates through a portion to be irradiated due to heat or in the case where printing is performed at a low laser output so as not to cause penetration of the light therethrough, the blackness is insufficient and a problem occurs such that marking with favorable visibility cannot be obtained.
PTL 3 describes that laser marking can be performed with a polymeric substance to which copper hydroxide monophosphate or molybdenum oxide is added, and describes that marking is performed by converting the additive to a colored product by a laser light. In the case where the additive itself is converted to a colored product by a laser light in this manner, the above-mentioned problems may be able to be avoided. However, fine printing cannot be performed, and also the blackness of the print is not a satisfactory level.
PTL 4 and PTL 5 describe that a resin composition and an ink containing bismuth oxide enable black marking by irradiation with a laser light.
However, in the case where laser marking is applied to a resin molded material containing an additive described in these patent literatures, the effect of improving the visibility is observed to some extent, however, the marking property such as blackness and contrast of the marking is still not satisfactory.
On the other hand, as described PTL 6, the present inventors found that a complex oxide composed of copper and molybdenum is discolored to have a color tone with high blackness by irradiation with a laser light. This complex oxide well absorbs a laser light and at the same time, the particles themselves are discolored from light yellow to black, and therefore, a resin molded material containing this oxide as an additive enables laser marking excellent in both blackness of marking and printing definition. However, the complex oxide has an undesirable problem that the oxide itself is yellow, and therefore, the resin molded material itself is slightly colored.
The present inventors further found in PTL 7 that a complex hydroxide composed of copper and molybdenum is discolored to have a color tone with higher blackness and printing definition by irradiation with a laser light, and also has a low resin coloration property. The complex hydroxide has excellent printing definition, however, particularly, in the case where laser marking is applied to an ink film with a small film thickness of 5 μm or less, an undesirable problem arises such that the marking property such as blackness and contrast of the marking is poor.